The present invention relates to highly durable, yet easily removable multi-layer protective coatings and their method of preparation. A conventional easily removable coating is first applied to a substrate, followed by application of a second coating that is highly durable and, in itself, not readily removable. The resultant multi-layer coating compositions of the present invention are particularly useful in polish and coating formulations where the combination of detergent resistance (durability) and easy removability properties is desirable.
Conventional highly crosslinked coating compositions typically do not provide easily removable coatings using traditional stripping operations (for example, treatment with solvent and ammonia or amines). Highly crosslinked coatings are desirable because of enhanced durability characteristics (such as long wear and scuff resistance) and are typically based on two-component (reactive) systems or systems requiring external energy sources to induce crosslinking (UV, electron beam) such that the resultant coatings do not swell significantly when exposed to typical chemical swelling agents and the film integrity is not disrupted to a sufficient extent to allow ready removability from a substrate. Typically, in order to assure enhanced durability, reduced ease of removability has become an undesirable but accepted characteristic of the highly durable coating compositions. For example, UV-curable coating compositions, such as those disclosed in U.S. Pat. No. 5,306,744 are extremely durable, but not readily removable by conventional stripping treatments. Similarly, single-package crosslinkable coating formulations based on polyurethane type and other latent crosslinking mechanisms (such as multicomponent systems) possess enhanced durability characteristics (scuff, mar or scratch resistance), but without the advantage of being readily removable upon demand. In addition, some of the crosslinking and post-crosslinking approaches suffer from other deficiencies, such as odor, environmental contamination, toxicity and compatibility problems. Due to their inherent lack of easy removability these highly durable coatings are typically limited in use to factory applied coatings and special end user markets, for example high performance wood floor coatings and industrial maintenance coatings.
Conventional easily removable protective coatings in the form of polishes are typically intended as sacrificial coatings to protect an underlying substrate by accepting and resisting marks, soils, scuffs, abrasion and scratches encountered in the normal use of the substrate, and, when the useful or aesthetic life of the protective coating has expired, the polish can be easily removed from the substrate to be replaced with a new coating. Typically, removability has been provided for these floor polishes by (i) incorporating metal ion crosslinking agents into polymers containing an excess of free carboxyl groups such that the metal ion crosslinking agents react with residual carboxyl groups (for example, from polymerized acrylic or methacrylic acid) or by (ii) the use of high levels of acid functionalized Alkali Soluble Resins (ASR) as formulation adjuncts. The relative excess of free carboxyl groups allows these coating compositions to swell when exposed to chemical swelling agents (such as aqueous ammonia or amines), thus rendering the coating easily removable when exposed to the stripping process; the swelling phenomenon interferes with cohesion and adhesion of the coating to the substrate such that film integrity is disrupted, thus facilitating removal of the coating from the substrate, for example, hard surfaces such as flooring, ceiling, walls and tiles. However, if sufficient acid functionality is added to the polish polymer or polish formulation for adequate long-term removability, resistance or durability of the polish film to scrubbing with alkaline detergent solutions is significantly diminished. Alternatively, if the acid functionality in the polish polymer or the amount of ASR in the formulation is reduced in order to allow for aggressive cleaning operations, then long-term ease of removability is compromised.
It would be desirable to provide coating compositions having the advantages of both the enhanced durability of highly crosslinked coatings and the easy removability of the more lightly crosslinked, but less durable, coating compositions. The problem addressed by the present invention is to provide coating compositions having these combined advantages, previously unavailable in conventional coating compositions.